SockJs client: add an XhrTransport for Undertow

This change adds a new XhrTransport for the SockJs client
implementation. This transport is based on `UndertowClient`,
Undertow's HTTP client.

Note that this transport can be customized with an OptionMap that is
used by the Xnio worker backing the I/O communications (see
http://xnio.jboss.org).

Issue: SPR-12008